The invention in the present application relates to a novel method of producing silicon carbide (SiC). The method is particularly useful for producing silicon carbide heating and lighting elements, high-temperature sensor elements, and finely-divided particles of silicon carbide, (e.g., for use as abrasives, for hardening surfaces, etc.), but can also be used for producing silicon carbide for many other applications, such as semi-conductor substrates, hard coatings for turbine blades, high power switching devices, cosmic radiation protectors, etc. The present application is directed to the production of heating and lighting elements of silicon carbide (SiC).
Silicon carbide (SiC), sometimes referred to as carborundum, is a hard, clear, green-tinged or yellow-tinged crystalline compound, which is normally insulating but which becomes conductive when properly heated at a high temperature; for example, when heated to 2000xc2x0 C., it is as conductive as graphite. This material, therefore, is frequently classified as a semiconductor. It is presently used in a wide variety of applications, including abrasives, heating elements, illuminating elements, high-temperature sensors and semiconductor substrates. Because of its highly unique properties, particularly hardness, heat resistance, semiconductivity, thermal and electrical stability, and corrosion resistance, it is commonly considered as the material of the future.
Silicon carbide is generally manufactured, according to one known method, by heating pure silica sand and carbon in the form of coke in an electrical furnace.
According to another known method, a graphite heating element in a cylinder bar is covered with mixture of carbon powder and quartz and high electrical current is passed through it to create a temperature of up to 3000xc2x0 C. At this temperature, the quartz (SiO2) is broken down to pure silicon, which reacts with the carbon powder and creates the required SiC. At a lower temperature zone, a distance from the heater, the SiC begins crystallizing in the shape of small scales. These scales are ground to form a powder of the required size. This process of SiC powder synthesis which takes place in a vacuum (10xe2x88x923 Torr), requires in the order of 36 hours, as well as high electrical currents. Moreover, it is difficult to obtain a powder of the required grain size with this process.
Approximately 45 years ago a new concept was proposed by Lely for growing silicon carbide crystals of high quality; and approximately 20 years ago, a seeded sublimation growth technique was developed (sometimes referred to as the xe2x80x9cmodified Lely Techniquexe2x80x9d). The latter technique lead to the possibility for true bulk crystal preparation.
However, these techniques are also relatively expensive and time-consuming, such that they impose serious limitations on the industrial potential of this remarkable material. In addition, silicon carbide heating or lighting elements prepared in accordance with these known techniques generally vary in resistance with temperature, and/or lose power with age, thereby requiring extra controls, special compensations, and/or frequent replacement.
An object of the invention in the present application is to provide a new method of producing silicon carbide heating and lighting elements having advantages in one or more of the above respects.
According to a broad aspect of the present invention, there is provided a method of producing silicon carbide (SiC) heating and lighting elements, comprising: mixing a quantity of finely-divided particles of carbon in a binder; applying finely-divided particles of elemental silicon over the carbon particles in the binder; and heating the silicon and the carbon in the binder, while subjected to a vacuum, to vaporize and diffuse the silicon and to react the silicon vapor with the carbon in the binder, to convert the carbon to silicon carbide; the silicon particles, when applied over the carbon particles in the binder, including a dopant to reduce the internal resistance of the produced silicon carbide to a value of up to a few hundred Ohm-cms such as to make the produced silicon carbide suitable as a heating or lighting element; the dopant including an element from the third or fifth column in the periodic table.
By elemental silicon is meant the silicon element, as distinguished from the silicon dioxide compound (e.g., sand, glass, quartz). Preferably, the silicon is relatively pure except for possible traces of impurities or dopants, such as present in silicon semiconductor substrates. In fact particularly good results were obtained, as described below, when the silicon used was the wastage in the manufacture of silicon semiconductor substrates.
Preferably, the carbon is either lignite carbon or anthracite carbon ground to a fine talc or power form.
During this heating process, the silicon vaporizes, diffuses into the carbon, and converts it to silicon carbide (SiC). Silicon carbide including such dopants has a green-tinged color, and therefore the formation of such a color during the above-described heating process indicates that the resulting product is indeed silicon carbide.
Since the novel method utilizes elemental silicon, rather than SiO2 (as in sand, glass or quartz), it does not require the high temperatures (e.g., the order of 3000xc2x0 C.), or the long heating time (e.g., the order of 36 hours) required on the prior art process as described above.
The method may be used in a wide variety of applications for producing various shaped articles of silicon carbide. The present application relates primarily to the production of silicon carbide heating and /or lighting elements.
In the preferred embodiments of the invention described below, the quantity of silicon is in excess of the quantity of carbon by weight to assure relatively complete conversion of the carbon to silicon carbide, with the excess silicon being removed by removing the silicon vapors during the diffusion process to prevent or minimize condensation of the silicon vapor on the outer surface of the silicon carbide.
Where heating or lighting elements are to be produced, the initial composition preferably includes relatively pure silicon but having traces of a dopant, such as zinc, aluminum, tellurium, or another element in the third or fifth column of the periodic table, in the ratio of about 1:10xe2x88x926. The vacuum is preferably from 10xe2x88x921 to 10xe2x88x923 Torr, and the heating temperature is preferably 1550-1600xc2x0 C. Such a process produces silicon carbon heating (or lighting) elements which are green-tinged in color, and have a relatively low internal resistance in the order of tens to a few hundreds of Ohm-cm.
In some described preferred embodiments, the mixture is prepared by mixing the finely-divided particles of carbon in a water solution of sucrose, and in other described preferred embodiments, the mixture is prepared by mixing the finely-divided particles of carbon in polyvinyl acetate. In both cases, the carbon mixture is prebaked at about 500xc2x0 C. in order to harden the sample. It will be appreciated, however, that other binders may be used.
According to further features in the described preferred embodiments, the carbon and silicon are both contained in a graphite crucible when heated within the furnace. The crucible is at least partly open at its upper end to the interior of the furnace to permit excess silicon vapors to escape to the interior of the furnace, and thereby to prevent or minimize condensation of silicon vapors on the outer surface of the silicon carbide.
Further features and advantages of the invention will be apparent from the description below.